The US military said it halted an air raid against Islamic State (IS) in eastern Syria after being told by Russia that it might have struck Syrian government forces.

The US Central Command statement came after Syria and Russia said the US-led coalition had struck a Syrian military base in Deir el-Zour which is surrounded by IS militants, enabling the extremists to advance.

The statement said: "The air strike was halted immediately when coalition officials were informed by Russian officials that it was possible the personnel and vehicles targeted were part of the Syrian military."

It added that "coalition forces would not intentionally strike a known Syrian military unit".

The US is not known to have directly struck Syrian president Bashar Assad's forces at any point during the five-year civil war.

It is unclear why coalition air forces would be mounting attacks during a fragile ceasefire which the US has worked to put in place. However, the ceasefire does not apply to attacks on IS.

A Russian defence ministry official said Syria has informed them that 62 of its soldiers were killed in the air strike. Russia has been waging a year-old air campaign on behalf of Syrian president Bashar Assad's forces and closely coordinates with them.

Defence ministry spokesman Maj Gen Igor Konashenkov said the air strike near Deir el-Zour airport was conducted by two F-16s and two A-10s. He did not identify the planes' country affiliation, but said they were part of the international coalition.

Maj Gen Konashenkov said Syrian authorities reported another 100 wounded, and that the planes came from the direction of the Syrian border with Iraq.

He said IS militants surrounding the air base launched an attack on the Syrian army positions after the air strike. He added that if the coalition attack was launched by mistake, the reason for it was a "stubborn reluctance by the American side to coordinate its action against terrorist groups in Syria with Russia".

IS has repeatedly attacked the government-held air base, which is an isolated enclave deep in extremist-held territory.

The Syrian military said the air strikes enabled an IS advance on a hill overlooking the air base. It called the strike a "serious and blatant attack on Syria and its military", and "firm proof of the US support of Daesh (IS) and other terrorist groups".

Mr Assad's government views all those fighting against it as "terrorists," and has long accused the US and other rebel supporters of backing extremists.

The US-led coalition has carried out thousands of air strikes against IS in Syria and Iraq over the past two years, allowing allied forces on the ground to liberate several towns and cities from the extremist group. Russia also carries out attacks against IS targets, in Deir el-Zour and other parts of Syria.

The ceasefire took effect on Monday, and despite reports of violations, it has largely held. However, aid convoys have been unable to enter rebel-held parts of the northern city of Aleppo - a key component of the deal.

Earlier on Saturday, Russian president Vladimir Putin questioned the US commitment to the ceasefire, suggesting that Washington was not prepared to break with "terrorist elements" battling Assad's forces.

Russia has accused Washington of failing to rein in the rebels, and on Saturday Mr Putin asked why the United States has insisted on not releasing a written copy of the agreement.

Officials have provided details of the agreement in press conferences, but have not released an official document, fuelling suspicions on both sides.

Mr Putin said during a trip to Kyrgyzstan: "This comes from the problems the US is facing on the Syrian track - they still cannot separate the so-called healthy part of the opposition from the half-criminal and terrorist elements.

"In my opinion, this comes from the desire to keep the combat potential in fighting the legitimate government of Bashar Assad. But this is a very dangerous route."

He appeared to be referring to the Fatah al-Sham Front, an al-Qaida-linked group previously known as the Nusra Front, which is deeply embedded in rebel-held areas and fights alongside more moderate groups.

Russian foreign minister Sergey Lavrov echoed Putin's remarks during a phone call with US Secretary of State John Kerry. Mr Lavrov noted the "refusal by an array of illegal armed groups to join the ceasefire", and Washington's obligation to "separate units of the moderate opposition from terrorist groupings".

Under the ceasefire agreement, the US and Russia would work together to target the Fatah al-Sham Front, as well as IS, while Mr Assad's forces refrain from striking opposition-held areas.

But Washington has warned Russia that unless aid is delivered to Aleppo, it will not move ahead with the formation of the joint coordination centre.